tests: add integration test for create, update & delete FlatRateFulfillmentMethod#5964

mikemurray merged 4 commits intoreactioncommerce:release-3.0.0reactioncommerce/reaction:release-3.0.0from

trojanh:updateFlatRateFulfillmentMethod-testtrojanh/reaction:updateFlatRateFulfillmentMethod-testCopy head branch name to clipboard